sounds good thank you for your explanation!
but even if i print a cube with 1cm x 1cm x 1cm the y axis is still 0.2-0.3mm smaller than the x axis and that happens every time (note: the difference de or increase with the size of the object) so f.e. my screw holes get egg shaped
78.74 steps/mm means if you ask the X or Y axis to move 1mm the controller sends 78.74 steps to the servo. This seems very straightforward to me. I don't know how to make it clearer. If you want to increase the scale (print bigger) on an axis by 3% then you want to increase the steps by 3%.
But I don't recommend this. The steps/mm is worked out pretty carefully based on diameters of pulleys and if you mess with it, the software endstops might not work if you print something at the full 205mm size.
The shrinkage you see is caused by several effects. I'd have to see the part to know exactly what the problem is. But PLA shrinking by 3% sounds about right. .25/10mm (your x axis) is 2.5% shrinkage. .45/10 is 3% shrinkage. It sounds like you have identical shrinkage on both axes.
So alternatively you could scale up all parts by 2.5 to 3% every time you load a part in Cura.
But in practice I think you will find that there are many other factors that change the dimensions of parts. For example vertical holes through a part are always smaller due to 3 factors: PLA shrinkage, the pulling effect as it lays down a "string" of PLA around the outside circle pulls it inward and the fact that circles are done in CAD with a polygons and these are inscribed *inside* the circle. So less than 10 sided polygons noticably shrink the hole diameter.
The best solution is to always print a part twice - or print small test sections, then increase the sizes in your CAD to compensate. Unfortunately different brands and colors of plastic will need different compensations and the way you print (temperature, fan settings, speed) will also change these so it's not something that you can share with other people.
Link to post
Share on other sites